Iran has dramatically stepped up covert attempts to buy nuclear equipment over the last six months, often by using Chinese companies as fronts, according to a senior German industrialist.

Ralf Wirtz, whose company, Oerlikon Leybold Vacuum, makes pumps that can be used in uranium enrichment centrifuges, said that more than five years after the AQ Khan nuclear smuggling network was exposed by US and British intelligence the black market trade was on the rise again.

"In the last six months I have seen a considerable increase of procurement attempts which - as we are told by government authorities - are for a nuclear programme," Wirtz told the world's leading nuclear experts gathered yesterday at a Washington conference organised by the Carnegie Endowment for International Peace.

He said Iran was seeking to buy equipment for its nuclear programme using increasingly sophisticated methods. Instead of using trading companies as fronts, Wirtz said Iran had recently placed orders for sensitive equipment through engineering companies that have legitimate uses for it. He gave the example of a recent attempt to buy his company's pumps through a Chinese engineering firm.

"European government authorities were notified, one of which learned from the Chinese government that the pumps did indeed go to Iran," Wirtz said. "Although they did not learn the exact end user, they believed Iran's centrifuge programme was the likely customer."

He said the Iranian use of Chinese companies was a growing trend.

Wirtz added that the firm in this case was probably not aware it was being used to further Iran's nuclear programme. However, another Chinese company was charged in a New York court yesterday with knowingly selling missile and nuclear technology to Iran.

Li Feng Wei and his company were indicted on 118 counts of fraud and conspiracy to supply Iran with equipment banned under a UN embargo.

David Albright, the head of the Institute for Science and International Security, said: "China is a huge hole in the system. It needs help to implement its own controls."

Abdul Qadeer Khan, a Pakistani nuclear scientist who helped mastermind that country's covert weapons programme, admitted in 2004 running a smuggling scheme selling technology to rogue regimes. He was freed from house arrest by a Pakistani court in February.

Only a handful of the other members of his international network, which supplied enrichment and other nuclear technology to Libya, Iran and North Korea, were ever tried, and none are currently in jail.

Wirtz likened the nuclear smuggling network to a chain store. "If you close a few stores, if you shut down some affiliates, the remaining ones continue to operate. Even if you close the corporate headquarters, the shops may be able to survive."

He said other companies making dual-use equipment were also receiving a growing number of suspect purchase requests, but in many cases, particularly in the US, those companies were not passing the information on to the authorities for fear of investigation and possible prosecution.

"Whenever I mention suspicious inquiries, there is consent from my colleagues that they also do have such requests for quotation, and that they just file them or put them into the trash bin," he said. "Frankly I would also keep my big mouth shut if all I got in return for my good citizenship is problems with national authorities, while criminals like [the AQ Khan network] walk away as free men after a while."

AQ Khan network

Abdul Qadeer Khan is a nationalhero in Pakistan for his role in building the country's nuclear programme. But the interception of a ship in 2003 carrying uranium centrifuge parts for Libya led to the exposure of a globalsmuggling ring Khan had been running for years. In February 2004, the Pakistan government said Khan had confessed to providing nuclear technology to Iran, Libya and North Korea. He was placed under house arrest but was freed by a court in February. He claimed he had made his confession under duress. After threats that US aid to Islamabad would be withheld, the government reassured Washington he would remain under surveillance and some restrictions.